package edu.princeton.cs.algs4;

import edu.princeton.cs.stdlib.StdOut;
import edu.princeton.cs.stdlib.StdRandom;

/*************************************************************************
 * Compilation: javac EdgeWeightedDirectedCycle.java Execution: java
 * EdgeWeightedDirectedCycle V E F Dependencies: EdgeWeightedDigraph.java
 * DirectedEdge Stack.java
 * 
 * Finds a directed cycle in an edge-weighted digraph. Runs in O(E + V) time.
 * 
 * 
 *************************************************************************/

public class EdgeWeightedDirectedCycle {
	private boolean[] marked; // marked[v] = has vertex v been marked?
	private DirectedEdge[] edgeTo; // edgeTo[v] = previous edge on path to v
	private boolean[] onStack; // onStack[v] = is vertex on the stack?
	private Stack<DirectedEdge> cycle; // directed cycle (or null if no such
										// cycle)

	public EdgeWeightedDirectedCycle(EdgeWeightedDigraph G) {
		marked = new boolean[G.V()];
		onStack = new boolean[G.V()];
		edgeTo = new DirectedEdge[G.V()];
		for (int v = 0; v < G.V(); v++)
			if (!marked[v])
				dfs(G, v);

		// check that digraph has a cycle
		assert check(G);
	}

	// check that algorithm computes either the topological order or finds a
	// directed cycle
	private void dfs(EdgeWeightedDigraph G, int v) {
		onStack[v] = true;
		marked[v] = true;
		for (DirectedEdge e : G.adj(v)) {
			int w = e.to();

			// short circuit if directed cycle found
			if (cycle != null)
				return;

			// found new vertex, so recur
			else if (!marked[w]) {
				edgeTo[w] = e;
				dfs(G, w);
			}

			// trace back directed cycle
			else if (onStack[w]) {
				cycle = new Stack<DirectedEdge>();
				while (e.from() != w) {
					cycle.push(e);
					e = edgeTo[e.from()];
				}
				cycle.push(e);
			}
		}

		onStack[v] = false;
	}

	public boolean hasCycle() {
		return cycle != null;
	}

	public Iterable<DirectedEdge> cycle() {
		return cycle;
	}

	// certify that digraph is either acyclic or has a directed cycle
	private boolean check(EdgeWeightedDigraph G) {

		// edge-weighted digraph is cyclic
		if (hasCycle()) {
			// verify cycle
			DirectedEdge first = null, last = null;
			for (DirectedEdge e : cycle()) {
				if (first == null)
					first = e;
				if (last != null) {
					if (last.to() != e.from()) {
						System.err
								.printf("cycle edges %s and %s not incident\n",
										last, e);
						return false;
					}
				}
				last = e;
			}

			if (last.to() != first.from()) {
				System.err.printf("cycle edges %s and %s not incident\n", last,
						first);
				return false;
			}
		}

		return true;
	}

	public static void main(String[] args) {

		// create random DAG with V vertices and E edges; then add F random
		// edges
		int V = Integer.parseInt(args[0]);
		int E = Integer.parseInt(args[1]);
		int F = Integer.parseInt(args[2]);
		EdgeWeightedDigraph G = new EdgeWeightedDigraph(V);
		int[] vertices = new int[V];
		for (int i = 0; i < V; i++)
			vertices[i] = i;
		StdRandom.shuffle(vertices);
		for (int i = 0; i < E; i++) {
			int v, w;
			do {
				v = StdRandom.uniform(V);
				w = StdRandom.uniform(V);
			} while (v >= w);
			double weight = Math.random();
			G.addEdge(new DirectedEdge(v, w, weight));
		}

		// add F extra edges
		for (int i = 0; i < F; i++) {
			int v = (int) (Math.random() * V);
			int w = (int) (Math.random() * V);
			double weight = Math.random();
			G.addEdge(new DirectedEdge(v, w, weight));
		}

		StdOut.println(G);

		// find a directed cycle
		EdgeWeightedDirectedCycle finder = new EdgeWeightedDirectedCycle(G);
		if (finder.hasCycle()) {
			StdOut.print("Cycle: ");
			for (DirectedEdge e : finder.cycle()) {
				StdOut.print(e + " ");
			}
			StdOut.println();
		}

		// or give topologial sort
		else {
			StdOut.println("No directed cycle");
		}
	}

}
